1. Field of the Invention
The present invention relates generally to an improved rear sole for footwear and, more particularly, to a rear sole for an athletic shoe with an extended and more versatile life and better performance in terms of cushioning and spring.
2. Discussion of the Related Art
Athletic shoes, such as those designed for running, tennis, basketball, cross-training, hiking, walking, and other forms of exercise, typically include a laminated sole attached to a soft and pliable upper. The laminated sole generally includes a resilient rubber outsole attached to a more resilient midsole usually made of polyurethane, ethylene vinyl acetate (EVA), or a rubber compound. When laminated, the sole is attached to the upper as a one-piece structure, with the rear sole being integral with the forward sole.
One of the principal problems associated with athletic shoes is outsole wear. A user rarely has a choice of running surfaces, and asphalt and other abrasive surfaces take a tremendous toll on the outsole. This problem is exacerbated by the fact that most pronounced outsole wear, on running shoes in particular, occurs principally in two places: the outer periphery of the heel and the ball of the foot, with heel wear being, by far, a more acute problem. In fact, the heel typically wears out much faster than the rest of the athletic shoe, thus requiring replacement of the entire shoe even though the bulk of the shoe is still in satisfactory condition.
Another problem associated with outsole wear is midsole compression. As previously noted, the midsole is generally made of a resilient material to provide cushioning for the user. However, after repeated use, the midsole is compressed due to the large forces exerted on it during use, thereby causing it to lose its cushioning effect. Midsole compression is the worst in the heel area, particularly the outer periphery of the heel and the area directly under the user's heel bone.
Despite technological advancements in recent years in midsole and outsole design and construction, the benefits of such advancements can still be largely negated, particularly in the heel area, by two months of regular use. The problems become costly for the user since athletic shoes are becoming more expensive each year, with some top-of-the-line models priced at over $150.00 a pair. By contrast with dress shoes, whose heels can be replaced at nominal cost over and over again, the heel area (midsole and outsole) of an athletic shoe cannot be. To date, there is nothing in the art to address the combined problems of midsole compression and outsole wear in athletic shoes, and these problems remain especially severe in the heel area of such shoes.
Designs are known that specify the replacement of the entire outsole of a shoe. Examples include those disclosed in U.S. Pat. Nos. 4,745,693, 4,377,042 and 4,267,650. These concepts are impractical for most applications, especially athletic shoes, for several reasons. First, tight adherence between the sole and the shoe is difficult to achieve, particularly around the periphery of the sole. Second, replacement of the entire sole is unnecessary based upon typical wear patterns in athletic shoes. Third, replacing an entire sole is or would be more expensive than replacing simply the worn elements, a factor which is compounded if a replaceable, full-length sole for every men's and women's shoe size is to be produced. Finally, it would appear that the heel section, in particular, has entirely different needs and requirements from the rest of the shoe sole and deteriorates at a much faster rate.
Other designs, which are principally directed to shoes having a relatively hard heel and outsole (e.g., dress shoes), disclose rear soles that are detachable and which can be rotated when a portion of the rear sole becomes worn. For example, U.S. Pat. No. 1,439,758 to Redman discloses a detachable rear sole that is secured to a heel of the shoe with a center screw that penetrates the bottom of the rear sole and which is screwed into the bottom of the heel of the shoe. Such a design cannot be used in athletic shoes because the resilient midsole and the soft, pliable upper are not rigid enough to retain the center screw. In addition, the center screw would detrimentally affect the cushioning properties of the resilient midsole and may possibly be forced into the heel of the user when the midsole is pressed during use.
Shoes with detachable rear soles that incorporate a center screw or other related securing means to attach the rear sole to the shoe also may experience gapping problems. Gapping refers to the gap that may appear, either initially or over time with extended use, between any detachable and non-detachable elements of a shoe. Any gapping will eventually attract debris or cause flapping and is otherwise aesthetically unpleasing. Such a problem would be particularly severe in a shoe that includes a rear sole made of resilient material that is likely to sag or move away from other surfaces with extended use. Similarly, rear soles dependent on center screws are likely to be pried away at the periphery when resilient materials are used. While related art discloses vertical heel support sidewalls, they do not solve either the gapping or the peripheral pry-away problem in the case of a resilient rear sole. For example, debris is still likely to lodge between a heel support vertical sidewall and a vertical rear sole sidewall; and the rear sole may still be pried away at the periphery if caught in a pavement crack or abrasion, if there is only a vertical wall to retain it. The latter problem is compounded by the fact that a vertical heel support sidewall would grip a resilient rear sole about its midsole where resiliency, by design, is the greatest and least able to resist displacement.
Rotating a rear sole will not, of course, counteract or alleviate midsole compression occurring at the heel center. While replacement of the entire rear sole is always an option, it may be that the full benefit of rotation will not have been realized when heel-center compression makes that necessary or desirable. That is to say that there may be good peripheral outsole and midsole remaining.
Although never in combination with a rotating or removable rear sole, there have been attempts to deal with heel-center midsole compression and/or to add spring to the user's gait by introducing various mechanical components into heel construction. One approach has been to insert horizontally in the heel area a thin layer of hard, flexible material that bends under the user's weight and then returns to its original position when the weight of the user is shifted to the other foot. Such attempts have met with only minimal success, however, for several reasons. Such insert may have lacked enough inherent resiliency from the outset. In other cases, it may have deteriorated with use. In all cases, it has rested on a resilient foundation around its periphery, limiting its ability to flex in the center.
Another problem is that athletic shoe purchasers cannot customize the cushioning or spring in the heel of a shoe to their own body weight, personal preference, or need. They are "stuck" with whatever a manufacturer happens to provide in their shoe size.
Finally, there appears to be relatively few, if any, footwear options available to those persons suffering from foot or leg irregularities, foot or leg injuries, and legs of different lengths, among other things, where there is a need for the left and right rear soles to be of a different height and/or different cushioning or spring properties. Presently, such options appear to include only custom-made shoes that are rendered useless if the person's condition improves or deteriorates.